In Burp Suite Enterprise Edition, "agents" are the virtual entities that perform your automated scans using their own embedded instance of Burp Scanner. Each agent can only perform one scan at a time. Therefore, to perform multiple scans simultaneously, you need multiple agents. When a scan is due to start, it is automatically performed by the next available agent.
The number of agents you can use depends on your license. Burp Suite Enterprise Edition's completely flexible scalability is based on the fact that there is no minimum or maximum number of agents you can have. Therefore, you only need to pay for the exact number of agents that you need. You can increase that number in as large or small increments as your needs dictate. If you are using a trial license, you are automatically licensed for 30 agents by default. If you are using a full license, you selected how many agents you wanted during the purchasing process. You can purchase a license for additional agents at any time.
Before you can perform any scans, you need to distribute your licensed agents across one or more agent machines on which they should run. You do this by assigning the maximum number of agents that are allowed to run on each machine. Burp Suite Enterprise Edition then allocates scans to agents accordingly when they are due to start.